Method and apparatus for power line communication

ABSTRACT

The variation cycle L of the characteristics of a transmission line is divided into a plurality of sections (n sections), a procedure is repeated in which transmission line estimation is performed for only one section among n sections in one beacon period, and thus transmission line estimation is performed for all of the n sections. The beacon period T is set based on (T=L×m/n), where n is an integer that is 2 or larger, and m is an integer that is n or larger and whose greatest common measure with n is 1.

TECHNICAL FIELD

The present invention relates to a communication apparatus and atransmission line estimation method. More specifically, the presentinvention relates to a communication apparatus sending and receivingdata based on the characteristics of a transmission line betweenapparatuses without lowering the throughput, and a transmission lineestimation method (channel estimation) performed by the communicationapparatus, by which the characteristics of the transmission line areestimated and evaluated with a high precision.

BACKGROUND ART

In a communication method by which communication parameters such as asubcarrier and a modulation method used for transmission and receptionare determined based on estimation on the characteristics of atransmission line, it is important to precisely determine thecommunication parameters that are suitable for the characteristics ofthe transmission line in transmission. In particular, in a communicationsystem having attenuation characteristics that deeply depend on thefrequency (power line carrier communications having a power line as acommunication medium, for example), it is effective to use amulti-carrier transmission line method using a subcarrier and amodulation method that are suitable for the characteristics of thetransmission line

In a transmission line estimation method used in conventionalcommunication systems, a transmission line is estimated periodically orwhen the number of retransmissions due to communication errors exceeds aspecified value (considering that the characteristics of thetransmission line are deteriorated) Then, based on the result of thisestimation on the transmission line, new parameters are selected, anddata is sent or received. This technique has been disclosed in, forexample, JP2002-158675A.

However, in an environment in which the characteristics of thetransmission line vary periodically, the communication parametersselected when estimating the transmission line often do not suit for thecharacteristics of the transmission line when sending data if a timingof sending data is not synchronized with the periodical variation of thecharacteristics of the transmission line. Thus, in the above-describedconventional method, the maximum communication efficiency is not alwaysobtained even when the transmission line is estimated.

Thus, as a countermeasure for this problem, a following method has beenconventionally proposed.

First, the variation cycle of the characteristics of a transmission lineis synchronized with the frame period of a communication system, andthis variation cycle is divided into a plurality of sections. Next,within one frame period, the plurality of divided sections of thetransmission line are continuously estimated section by section. Then,as a result of the transmission line estimation, communicationparameters obtained in a section with the highest communicationefficiency are selected and then communications are performed. FIG. 12is a process sequence of this conventional method for estimating atransmission line.

However, in the conventional method shown in FIG. 12, there is theproblem that the transmission line is estimated continuously, and thusrequests to estimate the transmission line and their response messagesoccupy the transmission line and disturb communications of stream data,audio data, or other data that is supposed to be sent. Furthermore, inthis conventional method, a time from the starting point of the frameperiod of the communication system to the starting time of atransmission line estimation section is different for each frame period.As a result, when the band is guaranteed, for example, with timesharing, not only is scheduling for transmission line estimationcomplicated, but also arises a case in which a scheduling conditioncannot be satisfied.

DISCLOSURE OF THE INVENTION

Therefore, an object of the present invention is to provide acommunication apparatus in which a transmission line is estimated in adistributed manner by a simple scheduling, so that the characteristicsof the transmission line are estimated and evaluated with a highprecision and thus data can be sent and received at a high throughputwithout affecting other streams, and a transmission line estimationmethod performed by the communication apparatus.

The present invention is directed to a communication apparatusperforming periodical communications with another communicationapparatus via a transmission line. In order to achieve theabove-described object, the communication apparatus of the presentinvention is provided with a communication control portion, atransmission line estimation portion, and a communication parameterdetermination portion.

The communication control portion sets the communication period to(L×m/n) (L is the variation cycle of the characteristics of atransmission line, n is an integer that is 2 or larger, and m is aninteger that is n or larger and whose greatest common measure with nis 1) to perform communications. The transmission line estimationportion estimates the characteristics of the transmission line within atime (L/n) after a certain offset time (L×k/n) (k is a real number thatis 0 or larger) has passed since the communication period started. Thecommunication parameter determination portion determines a communicationparameter to be used by the communication control portion, based on aresult of estimation by the transmission line estimation portion.

It is preferable that the transmission line estimation portion estimatesthe characteristics of the transmission line at least n times.Furthermore, the communication apparatus may estimate thecharacteristics of the transmission line at the initial starting up orupon detecting a change in a state of the transmission line. A typicalcommunication period is the period of beacons sent from a communicationapparatus serving as a base unit. When there is a request to estimatethe characteristics of the transmission line, the communicationapparatus sends a request to allocate a time for estimating thecharacteristics of the transmission line to the communication apparatusserving as the base unit, and the characteristics of the transmissionline are estimated only when permission is given. This request may benotified using the beacon frame or the polling frame to anothercommunication apparatus. A typical variation cycle L of thecharacteristics of the transmission line is the half cycle of thecommercial power supply cycle.

Each of the processes performed by each of the components of thecommunication apparatus described above can be regarded as atransmission line estimation method that gives a series of procedures.This method is provided in the form of a program for letting a computerexecute the series of procedures. This program may be introduced in acomputer in the form stored in a computer-readable storage medium.Furthermore, a part of the functional blocks described above thatconstitute the communication apparatus may be realized as an LSI, whichis an integrated circuit.

As described above, according to the present invention, a transmissionline is estimated in a distributed manner by a simple scheduling, sothat the characteristics of the transmission line are estimated andevaluated with a high precision and thus data can be sent and receivedat a high throughput without affecting other streams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration example of a communicationnetwork system using communication apparatuses according to a firstembodiment of the present invention.

FIG. 2 is a diagram showing an example of timings at which atransmission line is estimated by the communication apparatusesaccording to the first embodiment of the present invention.

FIG. 3 is a diagram showing another example of timings at which atransmission line is estimated by the communication apparatusesaccording to the first embodiment of the present invention.

FIG. 4 is a diagram showing another example of timings at which atransmission line is estimated by the communication apparatusesaccording to the first embodiment of the present invention.

FIG. 5 is a communication sequence showing the procedure following whicha transmission line is estimated by the communication apparatusesaccording to the first embodiment of the present invention.

FIG. 6 is a diagram showing an example of a tonemap.

FIG. 7 is a diagram explaining the relationship between a noise andtransmission line estimation sections.

FIG. 8 is another communication sequence showing the procedure followingwhich a transmission line is estimated by the communication apparatusesaccording to the first embodiment of the present invention.

FIG. 9 is a diagram explaining a method by which a beacon period isdetermined by the communication apparatuses according to the firstembodiment of the present invention.

FIG. 10 is a diagram explaining a method by which a beacon period isdetermined by the communication apparatuses according to the firstembodiment of the present invention.

FIG. 11 is a diagram showing an example of a communication networksystem in which the communication apparatuses of the present inventionare applied to high-speed power line transmission.

FIG. 12 is a communication sequence showing the procedure followingwhich a transmission line is estimated by a conventional communicationapparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

First Embodiment

FIG. 1 is a diagram showing a configuration example of a communicationnetwork system using a communication apparatus 1 according to a firstembodiment of the present invention. In FIG. 1 in the communicationnetwork system of the present invention, a plurality of communicationapparatuses 1 are connected to each other via a transmission line 2. Thetransmission line 2 may be either wired or wireless. This embodimentwill be described using, as an example, the communication network systemin which one of the plurality of communication apparatuses 1 is a masterunit, and this master unit periodically transmits a beacon so as tocontrol the communications of the other communication apparatuses 1(slave units).

The communication apparatus 1 is provided with a communication controlportion 11, a transmission line estimation portion 12, and acommunication parameter determination portion 13. The communicationcontrol portion 11 deals with most of the communication processesperformed by the communication apparatus 1. Basically, thiscommunication control portion 11 performs communications with anothercommunication apparatus 1 using communication parameters determined bythe communication parameter determination portion 13. The transmissionline estimation portion 12 measures the characteristics of thetransmission line 2 at predetermined periodical timings and estimates astate of the transmission line 2. The communication parameterdetermination portion 13 sets or updates communication parameters basedon the result obtained when transmission line estimation portion 12estimates the transmission line 2.

Hereinafter, a method by which the thus configured communicationapparatus 1 estimates the characteristics of a transmission line will bedescribed. FIGS. 2 to 4 are diagrams each showing an example of timingsat which a transmission line is estimated by the communication apparatus1 according to the first embodiment of the present invention. FIG. 5 isa communication sequence showing the procedure following which atransmission line is estimated by the communication apparatus 1according to the first embodiment of the present invention.

In this embodiment, a case will be described in which in thetransmission line 2 in the communication network system, a noise with acertain pattern (the mark X in FIG. 2) is generated with certainintervals as shown in FIG. 2, that is, the variation cycle of thecharacteristics of the transmission line corresponds to these certainintervals. In this case, the communication control portion 11 in each ofthe communication apparatuses 1 that constitute the communicationnetwork system sets the beacon period, which will be the communicationperiod, in the following manner. Herein, the beacon period refers to atime interval between when a beacon is transmitted by the master unitand when its next beacon is transmitted.

There is a case in which due to an influence of a power circuit of, forexample, a household electrical appliance that is connected to a powerline, the cycle of a noise pattern on the power line is the same as thehalf cycle of a commercial power supply (50 Hz or 60 Hz). Accordingly,when assuming a communication network system using a power line, it isnecessary to consider the characteristics of a transmission line thathas been synchronized with the half cycle of the commercial power supplydescribed above (see sine waves in FIG. 2).

A point of the setting is that a variation cycle L of thecharacteristics of the transmission line is divided into n sets ofsections (n sections), a procedure is repeated in which transmissionline estimation is performed for only one section among n sections inone beacon period, and thus transmission line estimation is performedfor all of the n sections. A beacon period T for realizing this point isset based on “T=L×m/n”, where n is an integer that is 2 or larger, and mis an integer that is n or larger and whose greatest common measure withn is 1. Furthermore an offset time is set based on “L×k/n”, where k is areal number that satisfies 0≦k<m. In this manner, when the offset timeis set to enable a transmission line to be estimated within one beaconperiod and section by section, it is possible to deal with a variationof the transmission line quickly. It should be noted that the offsettime can be set freely if dealing with a variation of the transmissionline rapidly is not considered.

FIGS. 2 and 3 are examples in which n=3 and m=17. FIG. 4 is an examplein which n=4 and m=19. When assuming a communication network systemusing a power line as described above, the beacon period T is calculatedusing L=8.333 msec when the commercial power supply frequency is 60 Hzand using L=10 msec when the commercial power supply frequency is 50 Hz.In FIGS. 2 to 4, a description of the beacon itself has been omitted.Furthermore, regarding the offset time, k=16 in FIG. 2, k=15 in FIG. 3,and k=17 in FIG. 4. As seen in FIGS. 2 to 4, when the beacon period andthe offset time are set based on the above-described points, after theoffset time has passed, each of the transmission line estimationsections does not have the same timing as that of the commercial powersupply cycle and slides therefrom. Therefore, it is possible to easilyrealize the transmission line estimation that does not overlap thecommercial power supply cycle and that is continuous in time.

Referring to FIG. 5, the procedure following which a transmission lineis estimated by the communication apparatus 1 will be described indetail.

At the initial starting up such as when the power is turned on, or upondetecting a change in the characteristics of a transmission line, acommunication apparatus 1 serving as a slave unit (hereinafter, referredto as apparatus A) requests a communication apparatus 1 serving as amaster unit (hereinafter, referred to as apparatus c) to allocate a timefor estimating the transmission line (step 1). When receiving therequest to allocate a time for estimating the transmission line from theapparatus A, the apparatus C sends a beacon, to which information ontime allocation for estimating the transmission line is added, duringthe next time of sending the beacon (step 2). This information on thetime allocation for estimating the transmission line refers toinformation showing sections that can be used for estimating thetransmission line, and is typically given as the offset time from thestarting time of the beacon period.

When receiving the beacon, to which the information on time allocationfor estimating the transmission line is added, from the apparatus c, theapparatus A measures the characteristics of the transmission line basedon this information, after the offset time has passed since the beaconperiod started. In the example in FIG. 2, the characteristics of thetransmission line are measured in a transmission line estimation section2/3 (the half-tone portion 2 in the drawing). A specific method formeasuring the characteristics of the transmission line is that theapparatus A sends a communication apparatus 1 serving as a slave unit ofinterest in the communications (hereinafter, referred to as apparatus B)a request to estimate the transmission line (step 3), and receives aresponse, from the apparatus B, for the request to estimate thetransmission line (step 4). This estimation on the transmission line is,for example, performed in the following manner.

First, a predetermined estimation series as well as the transmissionline estimation request are sent from the apparatus A to the apparatusB. Based on this estimation series, the apparatus B calculates thereceiving CNR (carrier to noise power ratio). Next, according to thecalculated receiving CNR, the apparatus B creates a tonemap thatspecifies communication parameters such as a subcarrier to be used and amodulation method for each subcarrier. An example of the tonemap isshown in FIG. 6. Herein, the tonemap is constituted by the tonemapnumber for discriminating this tonemap from other tonemaps, thesubcarrier number for identifying the subcarrier of interest in thistonemap, and information of use/non-use of the subcarrier and themodulation factor. The information on the modulation factor of thesubcarrier may be information on the modulation type (16 QAM or 32 QAM,for example) or may be the number of bit allocation of the subcarrier(“4” in the case of 16 QAM, for example) shown in FIG. 6. Then, theapparatus B responds to the apparatus A by sending the transmission lineestimation including the tonemap. It should be noted that theabove-described multi-carrier transmission method is an example, andother methods such as a spread spectrum method also can be used.Furthermore, the information on the receiving CNR is used fordetermining the communication parameters but information other than thisalso can be used.

With a similar process, the apparatus A measures the characteristics ofthe transmission line in the other transmission line estimation sections(steps 5 to 10). In the example in FIG. 2, the characteristics of thetransmission line are measured in a transmission line estimation section1/3 (the half-tone portion 1 in the drawing) and a transmission lineestimation section 3/3 (the half-tone portion 3 in the drawing). Withthis process, the apparatus A completes the transmission line estimationin all of these three divided transmission line estimation sections,that is, acquisition of the tonemaps (step 11). Then, among theplurality of acquired tonemaps, the apparatus A selects one tonemap thatis optimal for use in the communications, and notifies the apparatus Bof it (step 12). With this process, the apparatuses A and B can sharethe optimal tonemap. Hereinafter, the communications are performed usingthis optimal tonemap.

An optimal tonemap is selected, for example, in the following manner. Inthe example in FIG. 2, a noise is generated in the transmission lineestimation sections 1/3 and 2/3, and a noise is not generated in thetransmission line estimation section 3/3 (see FIG. 7 partiallyextracting and magnifying FIG. 2). Therefore, the tonemap of thetransmission line estimation section 3/3 has the highest PHY rate.Accordingly, this tonemap with the highest PHY rate is selected as thetonemap used for the communications.

Even without acquiring all of the tonemaps of the transmission lineestimation sections, it is possible to select an optimal tonemap amongtonemaps that have been acquired when a predetermined time-out periodhas passed.

As described above, according to the communication apparatus 1 of thefirst embodiment of the present invention, it is possible to easilyrealize transmission line estimation in a distributed manner. Thus, thecharacteristics of the transmission line are estimated and evaluatedwith a high precision, and thus data can be sent and received at a highthroughput.

In the first embodiment, the integers n and m are described as fixedvalues, but they can be changed dynamically in accordance with, forexample, a change in the transmission line based on the estimationresult of the transmission line, the value of the PHY rate, or thedegree of a variation of the PHY rate.

Furthermore, in the communication sequence shown in FIG. 5, a relevantapparatus other than the apparatuses A, B, and C is not described, butthe process is performed typically as shown in FIG. 8.

Referring to FIG. 8, information on time allocation for estimating thetransmission line is added to a beacon sent from the apparatus c. Theinformation on the time allocation for estimating the transmission linenot only specifies, for the apparatus A, a time during which a requestto estimate the transmission line can be sent to the apparatus B, butalso prohibits the apparatuses B, C, and others from transferring data.By prohibiting apparatuses other than that sends the request fromtransmitting in a time during which the request to estimate thetransmission line is sent (the half-tone period in FIG. 8), it ispossible to avoid a collision, for example, between the request toestimate the transmission line and data.

In a time during which a response to the request to estimate thetransmission line is sent from the apparatus B to the apparatus A, datatransfer by the apparatuses A, B, and others may be or may not beprohibited for the purpose of improving the throughput in the datatransfer. FIG. 8 is the communication sequence in which data transfer isnot prohibited. Herein, when data transfer is not prohibited, it ispreferable that the response from the apparatus B is given the highestpriority.

Furthermore, instead of a manner in which the receiver apparatus Bresponds to the transmitter apparatus A by sending the transmission lineestimation at each time as described above, the plurality oftransmission line estimations may be sent at one time, or the receiverapparatus B may select a tonemap based on the plurality of transmissionline estimations and notify the transmitter apparatus A of the selectedtonemap. In either case, the effect of the present invention is notlost.

Second Embodiment

The first embodiment described above is a technique assuming that thevariation cycle of the characteristics of the transmission line is knownin advance. Then, in a second embodiment below, a technique will bedescribed in which an optimal beacon period can be set automaticallyeven when the variation cycle of the characteristics of the transmissionline is not known in advance.

For example, a case will be described in which the communicationapparatus 1 can set both of a beacon period T1 (FIG. 9) when thevariation cycle L of the characteristics of the transmission line, whichis synchronized with a commercial power supply frequency of 60 Hz, is8.333 msec and when the integers n=3 and m=17, and a beacon period T2(FIG. 10) when the variation cycle L of the characteristics of thetransmission line, which is synchronized with a commercial power supplyfrequency of 50 Hz, is 10 msec and when the variables n=3 and m=17. Inthis case, the communication apparatus 1 estimates the transmission linein all of the identical sections within a beacon period and acquires aplurality of tonemaps. FIGS. 9 and 10 show a case of the transmissionline estimation section 2 (the half-tone portions in the drawings). Theoffset time is given based on “L×k/n” (k is a real number that satisfies0≦k<m), and k=16 in FIGS. 9 and 10.

Herein, it is assumed that the actual commercial power supply frequencyis 60 Hz.

As a result, the variation cycle L of the characteristics of thetransmission line in the beacon period T1 is synchronized more with anoise that is synchronized with the actual commercial power supplyfrequency (60 Hz) in the drawing (FIG. 9). Thus, the characteristics ofthe transmission line at the transmission line estimation sections 2 aresubstantially the same, and a similar value for the communicationparameter (PHY rate) of each tonemap can be obtained in the plurality ofacquired tonemaps, so that it is determined that the correlation,regarding a noise, between the beacon period T1 and the variation cycleL is high.

On the other hand, the variation cycle L of the characteristics of thetransmission line in the beacon period T2 is not synchronized with anoise that is synchronized with the actual commercial power supplyfrequency (60 Hz) in the drawing (FIG. 10). Thus, the characteristics ofthe transmission line at the transmission line estimation sections 2 aredifferent from each other, and the communication parameter (PHY rate) ofeach tonemap is different for each of the plurality of acquiredtonemaps. Therefore, it is determined that the correlation, regarding anoise, between the beacon period T2 and the variation cycle L is low.

Based on the above-described points, it is determined that the settingsof a beacon period determined to have the highest correlation issynchronized most with a noise that is actually being generated.Accordingly, only by selecting the settings of the beacon period withwhich the correlation is high, it is possible to automatically set abeacon period that corresponds to the variation cycle of thecharacteristics of the transmission line.

An optimal communication parameter that is noise-resistant is selectedby setting the beacon period according to the second embodiment and thenby performing the process according to the first embodiment.

The above-described embodiments can be realized also when a CPU executesa program that can let the CPU execute the above-described procedurestored in a storage device (ROM, RAM, or hard disk, for example). Inthis case, the program may be executed after being stored in the storagedevice via a storage medium, or may be executed directly on the storagemedium. The storage medium here includes a semiconductor memory such asa ROM, a RAM, and a flash memory, a magnetic disk memory such as aflexible disk and a hard disk, an optical disk such as a CD-ROM, a DVD,and a BD, and a memory card. Furthermore, the concept of the storagemedium also includes a communication medium such as a telephone line anda carrier line.

Each of the functional blocks indicated by the broken line in FIG. 1 maybe realized by an LSI, which is an integrated circuit. Each of thefunctional blocks may be formed on a single chip one by one, or a partor all of them may be formed on one chip. Although an LSI is used inthese embodiments, this circuit may be called IC, system LSI, super LSI,or ultra LSI, depending on the difference of the degree of integration.

It should be noted that the method for forming an integrated circuit isnot limited to using an LSI, and a circuit integration maybe realized bya dedicated circuit or a general purpose processor Furthermore, it ispossible to use an FPGA (field programmable gate array) that can beprogrammed after an LSI is produced, and a reconfigurable processorbeing capable of reconfiguring connections and settings of circuit cellsinside of the LSI.

Moreover, if circuit integration technology that replaces an LSI appearsdue to the development of semiconductor technology or derived anothertechnology, it is natural that the functional blocks may be integratedby using that technology. There is a possibility of, for example,application of biotechnology.

Hereinafter, an example will be described in which the present inventionthat has been described in the embodiments is applied to an actualnetwork system. FIG. 11 is a diagram showing an example of a networksystem in which the present invention is applied to high-speed powerline transmission. In FIG. 11, an IEEE1394 interface, a USB interface,or so forth provided in multimedia equipment such as a personalcomputer, a DVD recorder, a digital TV, and a home server system isconnected to a power line via an adaptor provided with the function ofthe present invention. With this configuration, it is possible toconstruct a network system in which digital data such as multimedia datacan be transmitted at a high speed via a power line serving as a medium.In contrast to a conventional wired LAN, this system does not require anetwork cable to be newly installed and can use a power line having beeninstalled already at home, office, or so forth without any process as anetwork line, so that a significant convenience in terms of cost andsimplicity of installation is provided.

The above-described embodiment is an example in which existingmultimedia equipment is applied to power line communications via anadaptor converting a signal interface of the existing equipment to aninterface of the power line communications. In future, however, thefunction of the present invention is included in multimedia equipment,so that it becomes possible to transmit data between the equipment viapower codes of the multimedia equipment. In this case there is no needfor the adaptor, the IEEE1394 cable, or the USB cable shown in FIG. 11,and thus wiring is simplified. Furthermore, since connection to theInternet via a router or connection to a wireless/wired LAN using, forexample, a hub is possible, so that it is possible to expand a LANsystem using the high-speed power line transmission system of thepresent invention. Furthermore, in the power line transmission method,communication data runs via a power line. Therefore, in contrast to awireless LAN, there is no problem that radio waves are intercepted,resulting in data leakage. The power line transmission method also has asecurity effect to protect data. Data running on the power line can beprotected by IPsec in IP protocol, encoding the contents themselves, orother DRM methods, for example.

As described above, by installing a QoS function including a copyrightprotection function by encoding the contents and the effect of thepresent invention (band allocation flexibly addressing improvement inthroughput, increased retransmission, and variation in traffic), AVcontents with a high quality can be transmitted via a power line.

INDUSTRIAL APPLICABILITY

The communication apparatus and the transmission line estimation methodof the present invention can be used, for example, in a communicationsystem in which the characteristics of a transmission line vary at acertain cycle, and are particularly useful, for example, when thecharacteristics of the transmission line are required to be estimatedand evaluated with a high precis ion so that data is sent and receivedat a high throughput.

1. A communication apparatus performing periodic communications withanother communication apparatus via a transmission line, comprising: acommunication control portion operable to set a communication period toL×m/n to perform communications, wherein L is a variation cycle ofcharacteristics of a transmission line, n is an integer that is 2 orlarger, and m is an integer that is greater than or equal to n and whosegreatest common measure with n is 1, a transmission line estimationportion operable to estimate the characteristics of the transmissionline within a time L/n after a certain offset time has passed since thecommunication period started, and a communication parameterdetermination portion operable to determine a communication parameter tobe used by the communication control portion, based on a result ofestimation by the transmission line estimation portion.
 2. Thecommunication apparatus according to claim 1, wherein the offset time isL×k/n, and k is a real number that satisfies 0≦k<m.
 3. The communicationapparatus according to claim 1, wherein the transmission line estimationportion estimates the characteristics of the transmission line at leastn times.
 4. The communication apparatus according to claim 1, whereinthe transmission line estimation portion estimates the characteristicsof the transmission line at an initial starting up of the communicationapparatus or upon a detection of a change in a state of the transmissionline.
 5. The communication apparatus according to claim 1, wherein thecommunication period is a period of beacons sent from a communicationapparatus serving as a master unit.
 6. The communication apparatusaccording to claim 5, wherein the communication control portion isoperable to send a request to allocate a time for estimating thecharacteristics of the transmission line to the communication apparatusserving as the master unit.
 7. The communication apparatus according toclaim 6, wherein the communication control portion is operable to notifyanother communication apparatus of an allocation of a time forestimating the characteristics of the transmission line using a beaconframe or a polling frame, and the transmission line estimation portionis operable to estimate the characteristics of the transmission lineonly when permission is given.
 8. The communication apparatus accordingto claim 1, wherein the variation cycle L of the characteristics of thetransmission line is a half cycle of a commercial power supply cycle. 9.A transmission line estimation method executed by a communicationapparatus performing periodic communications with another communicationapparatus via a transmission line, comprising: setting a communicationperiod to L×m/n to perform communications, wherein L is a variationcycle of characteristics of a transmission line, n is an integer that is2 or larger, and m is an integer that is greater than or equal to n andwhose greatest common measure with n is 1, estimating thecharacteristics of the transmission line within a time L/n after acertain offset time has passed since the communication period started,and determining a communication parameter to be used in thecommunications, based on a result of said estimating.
 10. An integratedcircuit used for a communication apparatus performing periodiccommunications with another communication apparatus via a transmissionline, wherein circuits are integrated that function as: a communicationcontrol portion operable to set a communication period to L×m/n toperform communications, wherein L is a variation cycle ofcharacteristics of a transmission line, n is an integer that is 2 orlarger, and m is an integer that is greater than or equal to n and whosegreatest common measure with n is 1, a transmission line estimationportion operable to estimate the characteristics of the transmissionline within a time L/n after a certain offset time has passed since thecommunication period started, and a communication parameterdetermination portion operable to determine a communication parameter tobe used by the communication control portion, based on a result ofestimation by the transmission line estimation portion.
 11. A beaconperiod determination method executed by a communication apparatusperforming periodical communications with another communicationapparatus via a transmission line, comprising: determining an offsettime based on one section among a plurality of sections which isobtained by dividing a variation cycle, and setting a beacon period thatis synchronized with a power supply cycle, based on the offset time. 12.The beacon period determination method according to claim 11, furthercomprising estimating, based on the offset time, characteristics of thetransmission line that is synchronized with the power supply cycle. 13.The beacon period determination method according to claim 11, furthercomprising: estimating, based on the offset time, characteristics of thetransmission line; determining a correlation, regarding a noise, betweenthe beacon period and the variation cycle, based on a tonemap obtainedfrom a result of estimation in the estimating step; and setting a beaconperiod that is determined to have high correlation.